Added check for last admin on role change

Will show error message if last admin and admin role is removed.
Closes #1124
Also cleaned up user controller a little.

resources/lang/en/errors.php

@@ -64,6 +64,7 @@ return [
     'role_cannot_be_edited' => 'This role cannot be edited',
     'role_system_cannot_be_deleted' => 'This role is a system role and cannot be deleted',
     'role_registration_default_cannot_delete' => 'This role cannot be deleted while set as the default registration role',
+    'role_cannot_remove_only_admin' => 'This user is the only user assigned to the administrator role. Assign the administrator role to another user before attempting to remove it here.',
 
     // Comments
     'comment_list' => 'An error occurred while fetching the comments.',